Measurement of the energy expenditure of humans is important for a number of reasons. For nutritional purposes measurement of the resting energy expenditure is important to a determination of the proper caloric content for feedings of hospitalized patients in view of the fact that certain diseases and traumas may cause the resting energy expenditure to vary substantially from normal values. In burn patients the metabolic rate may increase by as much as 300%. Other hospital situations in which the measurement of rate of metabolic oxygen consumption is important include the adjustment of parental feedings for infants, and the control of respiratory gases during surgical operations. The resting energy expenditure may also decrease substantially in the course of a weight loss diet, and knowledge of this basal energy requirement is important to the adjustment of caloric inputs in order to achieve a target loss. Similarly, knowledge of caloric consumption during exercise is useful for cardiac rehabilitation and athletic training.
A variety of indirect calorimeters for measuring oxygen consumption during respiration have been devised and are available commercially. These broadly include closed circuit devices wherein oxygen depleted during respiration is replenished from an oxygen source and the volume of replenishing oxygen is measured to determine respiratory oxygen consumption. A device of this type is disclosed in U.S. Pat. No. 4,753,245. Open circuit devices generally measure the volume of inhaled gas and the proportions of carbon dioxide and oxygen in exhaled gas to determine the respiratory oxygen consumption. Devices of this class are disclosed in U.S. Pat. Nos. 3,523,529, 4,619,269, 4,221,224 and 4,572,208.
All of these devices are relatively complex and expensive and require specially trained technicians for their operation. Their use has largely been limited to hospital settings for the adjustment of nutritional requirements for critically ill patients in intensive care units.
A potentially simpler and less expensive form of calorimeter would measure the inhaled gas volume, pass the exhaled gas over a carbon dioxide scrubber to remove the lung contributed CO.sub.2 from the exhaled gas and then measure the remaining gas volume. The difference between the two measured volumes would be a direct function of the respiratory oxygen consumption. However, because the exhaled gas has substantially different temperature and water vapor content than the inhaled gas, the volume measurements may grossly misestimate the actual oxygen consumption. Additionally, because such a device would measure variations in the relatively small differential between two large measurements, design of the device to attain a reasonable accuracy presents a problem.